This research was supported by the In-House Research and Development Program of the Korea Institute of Energy Research (KIER) (B9-2411) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2016R1D1A1B03934840).

1. Preparation of Mo-coated glass by DC magnetron sputtering
<ol>
	<li>Load cleaned glass substrates into a DC magnetron and pump down to below 4 x 10-6 Torr.</li>
	<li>Flow Ar gas and set the working pressure to 20 mTorr.</li>
	<li>Turn on plasma and increase the DC output power to 3 kW.</li>
	<li>After pre-sputtering of 3 min for target cleaning, begin the Mo deposition until the Mo film thickness reaches approximately 350 nm.</li>
	<li>Set the working pressure to 15 mTorr while maintaining the same output power (i.e., 3 kW).</li>
	<li>Resume the Mo deposition until the total thickness of Mo reaches approximately 750 nm.</li>
</ol>
2. CIGS absorber layer deposition by means of a three-stage coevaporation
<ol>
	<li>Load Mo-coated glass into a preheated co-evaporator under a vacuum lower than 5 x 10-6 Torr.</li>
	<li>Set the temperatures of In, Ga, and Se effusion cells yielding deposition rates of 2.5 &#197;/s, 1.3 &#197;/s, and 15 &#197;/s, respectively.
	<ol>
		<li>Check the deposition rates using the quartz crystal microbalance (QCM) technique. The deposition rates are dependent on the set temperature of effusion cells and the amount of materials in the effusion cells.</li>
	</ol>
	</li>
	<li>Begin to supply In, Ga and Se onto the Mo-coated glass to form a 1 &#956;m-thick (In,Ga)xSey precursor layer at the substrate temperature of 450 &#176;C. The deposition time is 15 min (namely, 1st stage).</li>
	<li>Stop the In and Ga supplies and increase the substrate temperature to 550 &#176;C.</li>
	<li>Begin to supply Cu (deposition rate: 1.5 &#197;/s) onto the (In,Ga)xSey precursor and continue until the Cu/(In + Ga) compositional ratio of the film reaches 1.15. Note that the Se deposition rate is maintained at 15 &#197;/s through the 2nd stage (namely, 2nd stage).</li>
	<li>Stop supplying Cu and evaporate In and Ga again with the same deposition rates as the 1st stage to finally form an approximately 2 &#956;m-thick CIGS film with Cu/(In+Ga) compositional ratio of 0.9. Maintain the Se deposition rate and substrate temperature at 15 &#197;/s and 550 &#176;C, respectively. The deposition time of this stage is 4 min (namely 3rd stage).</li>
	<li>In order to ensure a complete reaction, anneal the deposited CIGS film under ambient Se (15 &#197;/s) for 5 min at the substrate temperature of 550 &#176;C.</li>
	<li>Cool down the substrate temperature to 450 &#176;C under ambient Se (15 &#197;/s) and then unload the CIGS-deposited substrate when the substrate temperature is below 250 &#176;C.</li>
</ol>
3. Growth of the CdS buffer layer on the CIGS absorber layer using a chemical bath deposition (CBD) method
<ol>
	<li>Prepare the CdS reaction bath solution in a 250 mL beaker by adding 97 mL of DI water, 0.079 g of Cd(CH3COO)2&#183;2H2O, 0.041 g of NH2CSNH2, and 0.155 g of CH3COONH4. Stir the solution for several minutes to mix. Make sure that all added solutes are completely dissolved.</li>
	<li>Add 3 mL of NH4OH (28% NH3) into the bath solution and stir the solution for 2 min. Figure 1 shows the experimental setup of CBD for CdS.</li>
	<li>Put the CIGS sample into the reaction bath solution using a Teflon sample holder.</li>
	<li>Put the reaction bath into the water-heat bath maintained at 65 &#176;C and stir the reaction bath solution at 200 rpm using a magnetic bar during the deposition process.</li>
	<li>React for 20 min to generate an approximately 70 to 80 nm CdS buffer layer on the CIGS.</li>
	<li>After the reaction, remove the sample from the reaction bath, wash with a flow of DI water, and dry with N2 gas.</li>
	<li>Anneal the sample at 120 &#176;C for 30 min on a hot plate.</li>
</ol>
4. Fabrication of the AgNW TCE network
<ol>
	<li>Prepare a diluted AgNW dispersion (1 mg/mL) by mixing 19 mL of ethanol with 1 mL of a purchased ethanol-based AgNW dispersion (20 mg/mL).</li>
	<li>Pour 0.2 mL of the diluted AgNW dispersion onto a CdS/CIGS sample (2.5 cm x 2.5 cm) to cover the whole surface of the sample and rotate the sample with 1,000 rpm for 30 s.</li>
	<li>Repeat step 4.2 as needed to achieve the desired optical and electrical properties. Spin-coat the AgNWs 3x. A scanning electron microscopy (SEM) image of spin-coated AgNW TCE is shown in Figure 2.</li>
	<li>After spin-coating, anneal the sample at 120 &#176;C for 5 min on a hot plate.</li>
</ol>
5. Deposition of the 2nd CdS layer
<ol>
	<li>Prepare a new CdS reaction bath solution as described in step 3.1.</li>
	<li>Deposit CdS as in section 3, except change the reaction time as necessary. 
	NOTE: We optimized the reaction time, and 10 min resulted in the CIGS device with the best performance. The effect of 2nd CdS deposition time on a CIGS thin film solar cell device performance can be found in our previous work14.</li>
</ol>
6. Characterization techniques
<ol>
	<li>Characterize the surface and cross-sectional morphology of AgNWs and CdS-coated AgNWs by field emission SEM and transmission electron microscopy (TEM).</li>
	<li>Measure solar cell performance using a current-voltage source equipped with a solar simulator (1,000 W/m2, AM1.5G).</li>
</ol>

<ol>
	<li>Lee, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+lateral+collection+length+of+charge+carriers+for+silver-nanowire-electrode-based+Cu(In,Ga)Se2+thin-film+solar+cells.">Determination of the lateral collection length of charge carriers for silver-nanowire-electrode-based Cu(In,Ga)Se2 thin-film solar cells.</a> Solar Energy. 180, 519-523 (2019).</li><li>Langley, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+transparent+conductive+materials+based+on+silver+nanowire+networks:+a+review.">Flexible transparent conductive materials based on silver nanowire networks: a review.</a> Nanotechnology. 24 (45), 452001 (2013).</li><li>Chung, C. -. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silver+nanowire+composite+window+layers+for+fully+solution-deposited+thin-film+photovoltaic+devices.">Silver nanowire composite window layers for fully solution-deposited thin-film photovoltaic devices.</a> Advanced Materials. 24 (40), 5499-5504 (2012).</li><li>Liu, C. -. H., Yu, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silver+nanowire-based+transparent,+flexible,+and+conductive+thin+film.">Silver nanowire-based transparent, flexible, and conductive thin film.</a> Nanoscale Research Letters. 6 (1), (2011).</li><li>Yu, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+flexible+silver+nanowire+electrodes+for+shape-memory+polymer+light-emitting+diodes.">Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes.</a> Advanced Materials. 23 (5), 664-668 (2011).</li><li>Chung, C. -. H., Hong, K. -. H., Lee, D. -. K., Yun, J. H., Yang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ordered+vacancy+compound+formation+by+controlling+element+redistribution+in+molecular-level+precursor+solution+processed+CuInSe2+thin+films.">Ordered vacancy compound formation by controlling element redistribution in molecular-level precursor solution processed CuInSe2 thin films.</a> Chemistry of Materials. 27 (21), 7244-7247 (2015).</li><li>Kim, A., Won, Y., Woo, K., Kim, C. -. H., Moon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+transparent+low+resistance+ZnO/Ag+Nanowire/ZnO+composite+electrode+for+thin+film+solar+cells.">Highly transparent low resistance ZnO/Ag Nanowire/ZnO composite electrode for thin film solar cells.</a> ACS Nano. 7 (2), 1081-1091 (2013).</li><li>Singh, M., Jiu, J., Sugahara, T., Suganuma, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin-film+copper+indium+gallium+selenide+solar+cell+based+on+low-temperature+all-printing+process.">Thin-film copper indium gallium selenide solar cell based on low-temperature all-printing process.</a> ACS Applied Materials and Interfaces. 6 (18), 16297-16303 (2014).</li><li>Kim, A., Won, Y., Woo, K., Jeong, S., Moon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-solution-processed+indium-free+transparent+composite+electrodes+based+on+Ag+Nanowire+and+Metal+Oxide+for+thin-film+solar+cells.">All-solution-processed indium-free transparent composite electrodes based on Ag Nanowire and Metal Oxide for thin-film solar cells.</a> Advanced Functional Materials. 24 (17), 2462-2471 (2014).</li><li>Shin, D., Kim, T., Ahn, B. T., Han, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solution-processed+Ag+Nanowires+++PEDOT:PSS+hybrid+electrode+for+Cu(In,Ga)Se2+thin-film+solar+cells.">Solution-processed Ag Nanowires + PEDOT:PSS hybrid electrode for Cu(In,Ga)Se2 thin-film solar cells.</a> ACS Applied Materials and Interfaces. 7 (24), 13557-13563 (2015).</li><li>Wang, M., Choy, K. -. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-nonvacuum-processed+CIGS+solar+cells+using+scalable+Ag+NWs/AZO-based+transparent+electrodes.">All-nonvacuum-processed CIGS solar cells using scalable Ag NWs/AZO-based transparent electrodes.</a> ACS Applied Materials and Interfaces. 8 (26), 16640-16648 (2016).</li><li>Jang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cu(In,Ga)Se2+thin+film+solar+cells+with+solution+processed+silver+nanowire+composite+window+layers:+buffer/window+junctions+and+their+effects.">Cu(In,Ga)Se2 thin film solar cells with solution processed silver nanowire composite window layers: buffer/window junctions and their effects.</a> Solar Energy Materials and Solar Cells. 170, 60-67 (2017).</li><li>Chung, C. -. H., Bob, B., Song, T. -. B., Yang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current-voltage+characteristics+of+fully+solution+processed+high+performance+CuIn(S,Se)2+solar+cells:+crossover+and+red+kink.">Current-voltage characteristics of fully solution processed high performance CuIn(S,Se)2 solar cells: crossover and red kink.</a> Solar Energy Materials and Solar Cells. 120, 642-646 (2014).</li><li>Lee, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robust+nanoscale+contact+of+silver+nanowire+electrodes+to+semiconductors+to+achieve+high+performance+chalcogenide+thin+film+solar+cells.">Robust nanoscale contact of silver nanowire electrodes to semiconductors to achieve high performance chalcogenide thin film solar cells.</a> Nano Energy. 53, 675-682 (2018).</li></ol>

The authors declare that they have no competing financial interests.

Note that the deposition time of the 2nd CdS layer must be optimized to achieve the optimal cell performance. As the deposition time increases, the thickness of the 2nd CdS layer increases, and consequently, the electrical contact will improve. However, further deposition of the 2nd CdS layer will result in a thicker layer that reduces light absorption, and the device efficiency will decrease. We achieved the best cell performance with 10 min of deposition time for the 2nd CdS ...

Silver nanowire (AgNW) networks have been extensively studied as an alternative to indium tin oxide (ITO) transparent conducting thin films due to their advantages over conventional transparent conducting oxides (TCOs) in terms of lower processing cost and better mechanical flexibility. Solution-processed AgNW network transparent conducting electrodes (TCEs) have thus been employed in Cu(In,Ga)Se2 (CIGS) thin-film solar cells1,2,3,4,5,6. Solution-processed AgNW TCEs are normally fabricated in the form of embedded-AgNW or sandwich-AgNW structures in a conductive matrix such as PEDOT:PSS, ITO, ZnO, etc.7,8,9,10,11 The matrix layers can enhance that the collection of the charge carriers present in the empty spaces of the AgNW network.
However, the matrix layers can generate interfacial defects between the matrix layer and underlying CdS buffer layer in CIGS thin-film solar cells12,13. The interfacial defects often cause a kink in the current density-voltage (J-V) curve, resulting in a low fill factor (FF) in the cell, which is detrimental to solar cell performance. We previously reported a method to resolve this issue by selectively depositing an additional thin CdS layer (2nd CdS layer) between the AgNWs and the CdS buffer layer14. The incorporation of an additional CdS layer enhanced the contact properties in the junction between the AgNW and CdS layers. Consequently, the carrier collection in the AgNW network was greatly improved, and the cell performance was enhanced. In this protocol, we describe the experimental procedure to fabricate robust electrical contact between the AgNW network and the CdS buffer layer using a 2nd CdS layer in a CIGS thin-film solar cell.

<table><tbody><tr><td>Mo</td><td>Materion</td><td>Purity: 3N5</td><td>Mo sputtering</td></tr><tr><td>Cu</td><td>5N Plus</td><td>Purity: 4N7</td><td>CIGS deposition</td></tr><tr><td>In</td><td>5N Plus</td><td>Purity: 5N</td><td>CIGS deposition</td></tr><tr><td>Ga</td><td>5N Plus</td><td>Purity: 5N</td><td>CIGS deposition</td></tr><tr><td>Se</td><td>5N Plus</td><td>Purity: 5N</td><td>CIGS deposition</td></tr><tr><td>Ammonium acetate</td><td>Alfa Aesar</td><td>11599</td><td>CdS reaction solution</td></tr><tr><td>Ammonium hydroxide</td><td>Alfa Aesar</td><td>L13168</td><td>CdS reaction solution</td></tr><tr><td>Cadmium acetate dihydrate</td><td>Sigma-Aldrich</td><td>289159</td><td>CdS reaction solution</td></tr><tr><td>Thiourea</td><td>Sigma-Aldrich</td><td>T8656</td><td>CdS reaction solution</td></tr><tr><td>Silver Nanowire</td><td>ACSMaterial</td><td>AgNW-L30</td><td>AgNW dispersion</td></tr></tbody></table>

fabrication of robust nanoscale contact between a silver nanowire electrode and cds buffer layer in cu(in,ga)se2 thin-film solar cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

The layer structures of the CIGS solar cells with (a) standard ZnO:Al/i-ZnO and (b) AgNW TCE are shown in Figure 3. The surface morphology of CIGS is rough, and a nanoscale gap can form between the AgNW layer and the underlying CdS buffer layer. As highlighted in Figure 3A, the 2nd CdS layer can be selectively deposited onto the nanoscale gap to create a stable electrical contact. The detailed explanation on the format...

Watch this Scientific Journal Video about Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells at JoVE.com

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes ...

fabrication-robust-nanoscale-contact-between-silver-nanowire

Research

JoVE Journal

Engineering

6.2K Views.  Hanbat National University. Silver nanowire netwerken zijn een opkomende technologie ter vervanging van traditionele transparante geleidende oxiden in de dunne-film zonnecel applicatie. Het elektrische contact met de onderliggende laag is echter een probleem geweest. Ons protocol is een eenvoudige procesmethode om elektrische contacteigendom tussen zilvernanowirenetwerk en de onderliggende CdS-bufferlaag in CIGS dunne-film zonnecellen te verbeteren.Onze methode is een zeer eenvoudig, reproduceerbaar en goedkoop o...

Video: Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Fabricating Nanogaps by Nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

The fabrication of electrically addressable, high-aspect-ratio (> 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

Chemistry

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of stretchable electronic devices is the preparation of stretchable conductors with great mechanical stability. In this study, we developed a simple fabrication method to chemically solder the contact points between silver nanowire (AgNW) networks. AgNW nanomesh was first deposited on a glass slide via spray coating method. A reactive ink composed of silver nanoparticle (AgNPs) precursors was applied over the spray coated AgNW thin films. After heating for 40 min, AgNPs were preferentially generated over the nanowire junctions to solder the AgNW nanomesh, and reinforced the conducting network. The chemically modified AgNW thin film was then transferred to polyurethane (PU) substrates by casting method. The soldered AgNW thin films on PU exhibited no obvious change in electrical conductivity under stretching or rolling process with elongation strains up to 120%.

A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 &#937;/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) &#8211; up to 1.5 x 104 &#8211; which are amongst the best values in the published literature.

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se₂ Thin-film Solar Cells

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